The present invention relates to the manufacture of optical fibers in general, and more particularly to a optical preforms from tubular bodies. '
There are already known various methods of, and arrangements for, producing optical fibers, most of which involve first the production of an optical preform and then the drawing of the optical fiber from the preform. One way of producing the optical preform is the modified chemical vapor deposition method, during the preformance of which a gaseous medium is caused to flow through the , interior of a tubular substrate body. The gaseous medium includes reactants which decompose or interreact or are otherwise chemically transformed when heated to transformation temperatures into solid glass constituents. The tubular substrate is locally heated from the outside and the heating zone is gradually moved longitudinally and/or circumferentially of the tubular substrate, resulting in transformation of the reactants into the glass constituents, precipitation of the glass constituents from the gaseous medium in the form of glass soot and deposition of such glass soot on the internal surface of the tubular substrate and subsequent fusing of the deposited glass soot with tne substrate and/or any previously deposited internal layers. The modified chemical vapor deposition process is conducted in a plurality of passes or increments. The composition of the gaseous medium can be changed from one pass to another so as to change the chemical composition and the optical properties of the consequtively deposited layers of glass. After the deposition operation is completed, the resulting glass formation, which is still tubular, is often caused to collapse into a solid cross-section optical glass preform, which is subsequently used in the optical fiber drawing operation.
It has been established that it is advantageous to maintain positive pressure in the interior of the tubular formation during the collapsing operation in order to maintain roundness of the formation as it is being collapsed, usually in plurality of increments or passes. Prior to the present invention, however, this has been usually achieved in a static manner, without any flow of gaseous medium through the internal passage of the tubular formation during the collapsing operation. Rather, continuous flow out of one end of the passage in the tubular formation was prevented in one way or another, and gaseous medium at the desired superatmospheric pressure was supplied to the other end of the passage, so that this desired superatmospheric pressure prevailed throughout the internal passage. Experience with this kind of an approach has shown, however, that the pressure differential between the exterior and the interior of the tubular formation fluctuates during the collapsing operation, which brings about irregularities in the collapsing process. Moreover, at the relatively high temperatures used during the collapsing process, a substantial proportion of dopants such as germania will evaporate from the innermost layers of the tubular formation, thus changing the optical properties of such layers. Thus, a pronounced dimple is produced in the reflective index characteristic of the core of the preform and thus of the fiber drawn from the latter, unless the affected layer or layers are removed by etching just prior to the last collapsing step. The presence of such a pronounced dimple will degrade the optical properties of the optical fiber, but the etching away of the innermost layer or layers is a time consuming operation so that it is preformed only in the manufacture of preforms for very high-quality optical fibers.